This invention relates to a device for use in controlled descent from, for example, higher stories of buildings in the event of a fire or other emergency, with the evacuee suspended therefrom by wire rope or equivalent means. More specifically the invention concerns such a descent retarder utilizing the principle of conversion of dynamic energy into heat by magnetically induced eddy currents for braking the descent of the evacuee or refugee.
Most of the retarders heretofore suggested and used for the purposes in question have relied upon frictional conversion of dynamic energy into heat energy. Retarders of this known type have difficulties in developing constant heat and, therefore, a constant braking force. Few of them, moreover, are suitable for use in escape from considerable heights above ground, such as the higher stories of a multistory building. Because of these limitations of the conventional frictional retarders, and with increasing numbers of human lives lost in fires taking place in multistory buildings, there has been awaited the advent of a more reliable device that stands on an entirely different principle of operation.
Some large size motor vehicles today are equipped with eddy current retarders as one of the auxiliary brakes that supplement, together with exhaust brakes or the like, the main service brake. An eddy current retarder with a diameter of approximately 50 centimeters that can develop a braking torque of as much as 90 kilogram meters is in actual use on some large size trucks and trailers. It is evident from this that an eddy current retarder lends itself to use in controlling the descent of human beings. For the power generated by a man weighing 100 kilograms to descend 100 meters at a rate of 1.3 meters per second is only 1.7 horsepowers or so.
However, the eddy current retarders in use on motor vehicles need some basic alterations for use for the purpose now under consideration. The greatest problem is the use of electromagnets in the motor vehicle retarders for inducing eddy currents, making the retarders bulky, heavy, and expensive. The use of electromagnets is a necessity in motor vehicle retarders as the braking force must be released or otherwise controlled. In descent retarders, however, no such release or any other intentional control of the braking force is required.